Luckily I am pretty familiar with Centrelink processes. First thing anyone still on payments should do is have the repayments reduced to $15 a fortnight. If even that will cause financial stress, they can suspend repayments, but I've read they've been told to resist that. That will revert after 3 months, when you'll have to ask again.
Also, if you want a copy of your data eg. how much you claimed, how much you got paid etc., you probably don't have to file a Freedom of Information request, you can ask for the data under Administrative Access provisions. I did this and they sent me the data. So now I'm only paying $15 a fortnight, and the debt is under appeal. I expect it will be overturned eventually. Because of all this I'm not too stressed about it.
I don't think this is a mistake. I think that this is purposeful to torture people on welfare and make it clearly unpleasant for anyone thinking of going on welfare. As well as possibly laying the groundwork for privatisation, it will make people more desperate for work, and hopefully (to the Govt and their mates) work for less, thereby making the bosses more money. This purposeful data mismatching to produce robodebts is the weaponisation of public service bureaucracy and has to stop. The Government clearly has no conscience.